The art of artificial tooth roots has a long history. Since as early as the 1930's, various attempts have been made for material development and improvement in configuration and implanting techniques. Various kinds of artificial tooth roots have hitherto been used including those made of metals (such titanium and Co - Cr alloy), alumina-ceramic, zirconia-ceramic, and sintered aphthite material. In configuration they are classified as screw type, hollow type, and blade type; and they have their respective merits and demerits. There are known two implanting methods, namely, single implanting method and double implanting method.
For the purpose of implanting an artificial tooth root in a jawbone and mucous structure and fixing it in position, it is today believed essential that if the artificial tooth root is to be long stabilized and kept in its implanted condition without being disturbed, there must be an early bonding between the artificial tooth root and the jawbone at their interface so that the osteobond is histologically fixed.
The condition for success in artificial tooth root implanting which has so far been made clear is that a good bond between the implanted portion of the artificial tooth root and the jawbone be achieved at an early point of time. Necessary factors for achievement of an osteobond include designing as to implanting procedure and configuration, and material selection. Most artificial tooth roots in use today relate to the single implant technique.
Artificial tooth roots of the screw type or the blade type, in which the pin neck portion is exposed in an oral cavity are used in conjunction with single implanting techniques in such a way that natural implantation is intended while the pin neck portion is kept in projection in the oral cavity to allow an osteobond. However, the difficulty is that occlusion with an opposite tooth is commenced before a sufficient bond is obtained between the implanted portion of the artificial root and the jawbone; therefore, some excessive pressure is exerted on the artificial tooth root during mastication, and inflammation is caused by lateral pressure due to tongue depression or by infection, it being thus impossible to obtain unstrained implantation, with the result of root shaking or falling off.
Another difficulty with such root implanted by single implant techniques is that just after extraction, the jawbone portion of the wound socket by which the root of a natural tooth root had been supported is a soft granulation tissue, and therefore that the artificial tooth root, if immediately planted in the socket, is easily disturbed and cannot be kept stable. Most artificial tooth roots known today are implanted after a post-tooth-extraction jawbone is well ossificated, but in this case the trouble is that since a jaw bank tends to shrink and absorb after teeth falling, it becomes gradually difficult to implant artificial tooth roots as ageing progresses. In this way, the single implanting technique involves many problems.
Therefore, if it is possible to employ a double implanting method such that a blade-type piece is first implanted in a jawbone and allowed to stand until an osteobond is completely formed between the blade type piece and the jawbone, and after completion of such bond a pin neck portion is implanted in the blade type piece, aforesaid problems can be solved. That is, in order to achieve good osteobond, it is desirable to employ a double implanting method wherein complete implantation is effected which provides freedom from disturbance.
An artificial tooth root which can be used with such double implanting technique is known, as typically found in Japanese Published Examined Patent Application No. 56-27262, wherein a piece implantable in a jawbone consists of a cylindrical portion and side arms, and wherein a fitting pin brought in thread engagement with a threaded hole defined in the cylindrical portion is disengagesd from the threaded hole, a pin neck portion being then brought into thread engagement with the threaded hole for implantation therein.
One drawback of this prior-art arrangement is that if the cylindrical portion is made diametrically large, it is difficult to implant it stably in a jawbone having a limited width, while on the other hand, if the cylindrical portion is made small in diameter, the threaded portion of a pin neck portion is made diametrically small correspondingly, it being thus unexpectable to obtain good strength enough to withstand any subtantial force resulting from occlusion with an opposite tooth. Another difficulty with the known arrangement, in which each side arm is formed with a slit to ensure strong osteobond with the jawbone, is that only with such slit it is difficult to expect any substantially strong osteobond. If an attempt is made to construct the side arm larger in order to obtain increased bond strength, it is impracticable to make the side arm to the required extent because of the presence of adjacent natural teeth or otherwise; and if the side arm is made large at all, it will have an adverse effect on adjacent teeth. Another difficulty is that the joint portion between the pin neck portion and an upper structure makes a filthy region of oral mucous membrane and is likely to invite inflammation.